This invention relates to a shaft sealing device for a hydraulic machine, and more particularly to such device of the type adapted for preventing water from leaking out along the shaft of a hydraulic machine such as a hydraulic turbine or pump.
A shaft sealing device including packings is used between a fixed structural member and a movable shaft rotatable about its axis or reciprocatable along its length, for sealing a space between the two machine and for preventing water or the like fluid from leaking out of the space.
In the above described application, although a pressure exerted radially inwardly to the outer surface of each packing is essential for ensuring the shaft pressure to each packing tends to restrict the movement of the shaft, accelerates wear of the packings and the shaft surface, and in the worst case, causes shaft seizure.
FIG. 1 shows one example of a conventional shaft sealing device applied to a hudraulic turbine, the latter comprising a spiral casing 1, a shaft 6, and a runner 3 secured to the shaft 6 to be rotatable in the casing 1. The pressurized water in the spiral casing 1 is passed through a guide vane 2 toward the runner 3 to drive the same, and the pressurized water exhausted is discharged through a draft tube (not shown) to a tail race. Simultaneously, a portion of the pressurized water is caused to flow upward through a gap between the runner 3 and the head cover 4 of the turbine into a space defined by the outer surfaces of the shaft 6 and the head cover 4. A shaft sealing device 7 is provided between the shaft 6 and the head cover 4 for preventing water from leaking out through a gap between the two members 6 and 4.
As shown in detail in FIG. 2, the shaft sealing device 7 comprises a packing box 9 secured to the head cover 4 by means of bolts 8, packings 10.sub.a, 10.sub.b, and 10.sub.c contained in the packing box 9 at positions vertically aligned along the shaft 6, retainers 11 provided respectively radially outwardly of the packings 10.sub.a, 10.sub.b, 10.sub.c and garter springs 12 respectively provided about the outer circumferences of the retainers 11 for urging the retainers 11 toward the shaft 6.
Among the above described packings 10.sub.a, 10.sub.b, and 10.sub.c, packings 10.sub.a and 10.sub.b provided at the upper side are carbon packings ordinarily containing furan and sintered, while a packing 10.sub.c provided at the lower side is a resin packing ordinarily made of phenolic resin.
Each of the packings 10.sub.a, 10.sub.b, and 10.sub.c is divided along its circumference into a plurality of packing pieces as shown in FIG. 3. Each packing piece has a projection 14 at a circumferential end and also a recess 15 at the other circumferential end thereof. When the packing pieces are arranged circumferentially about the shaft 6 so that the inner surfaces of the packing pieces are brought into contact with a sleeve 16 tightly fitted about the shaft 6, the projection 14 of one packing piece is received in the recess 15 of an adjacent packing piece as shown in the drawing, leaving a gap g between the opposing end surfaces.
A fresh water supply hole 17 is provided in the packing box 9 to pass through a portion thereof between the packings 10.sub.b and 10.sub.c. A water feed pipe 18 is connected with the hole 17 for supplying fresh water throughout the sliding surfaces of the packings between a flange 19 of the shaft 6 and the packing box 9, or a pressure higher than that of the chamber 20 by 1-2 kg/cm.sup.2.
By the supply of the pressurized fresh water, any possibility of mud or sand from flowing upwardly into the sealing device and the packings and the possibility of overheating and wearing can be substantially eliminated.
Above the packing box 9, there is provided a water splashing plate 21 which splashes the water which leaks through the packings away from the shaft and prevents the water from entering into a bearing 22 provided above the shaft sealing device 7.
However, in the above described conventional shaft sealing device utilizing carbon packings 10.sub.a and 10.sub.b and a resin packing 10.sub.c, it has been found that the upper limit of the shaft sealing pressure of the carbon packings 10.sub.a and 10.sub.b is approximately 5 kg/cm.sup.2, and the shaft sealing pressure of the resin packing 10.sub.c is less than 3 kg/cm.sup.2 per one stage of the packing. For this reason a great number of packing stages are required for a high-pressure hydraulic turbine, thus resulting in an increase in a distance L between the center of the runner 3 and the center of the bearing 22 as shown in FIG. 1.
The increase of the distance L increases the overhang of the shaft projecting downwardly beyond the bearing 22, thus increasing the degree of the shaft deflection thereof and causing the operation of the turbine to be unstable.
Furthermore, the conventional carbon packing is easily broken because of its brittleness and tends to be worn out by mud and sand entering into the shaft sealing device. Augmenting the section modulus of the carbon packing for the purpose of strengthening the same inevitably increases the size of the packings and hence the distance L between the runner and the bearing.